1. Field of the Invention
The present invention relates to molding compounds for electrical and electronic devices, particularly epoxy-based compounds exhibiting flame resistance, moisture resistance, and low warpage and shrinkage.
2. Brief Description of Related Technology
Epoxy resins are widely used in molding compounds for coating electrical and electronic devices. Such epoxy molding compounds used for encapsulation are generally prepared from a blend of an epoxy resin and phenol hardener, along with other ingredients including fillers, catalysts, flame-retardant materials, processing aids, and colorants. Epoxy resins in such molding compounds are traditionally diepoxides which include two epoxy groups per molecule, which are reacted with a co-reactant (crosslinking agent or hardener) consisting of acid dianhydride, diamine, or diphenol oligomers. Diphenol oligomers, such as those derived from novolac phenols, cresol phenols and bisphenol A, are particularly preferred in the art as hardeners due to their high reliability.
Flame-retardants in epoxy compositions are typically provided for safety purposes. A common flame-retardant system is a combination of bromine-containing flame-retardants and antimony oxide flame-retardant synergists. However, these compounds are pollutants of the environment. Some bromine-containing flame-retardants (especially brominated diphenyl ethers) are toxic and possibly carcinogenic. Antimony trioxide is classified by the International Agency for Research on Cancer as a Class 2B carcinogen (i.e., antimony trioxide is a suspect carcinogen based mainly on animal studies). In addition, this compound is often used at a relatively high level (2-4%) and is also slightly water-soluble, leading to further environmental concerns. This concern is highlighted by the fact that integrated circuit manufacturers currently discard up to one half of the total amount of molding compositions used.
Phosphorus-containing compounds have been proposed as flame-retardants. For example, U.S. Pat. No. 5,739,187 to Asano et al. discloses epoxy resin compositions as semiconductor encapsulants, which include a phosphorus-containing flame-retardant to eliminate the use of antimony trioxide and brominated compounds. However, molding compositions containing conventional phosphorus compounds generally possess undesirable properties such as high moisture absorption, which can cause stress and cracking of the encapsulant at elevated temperatures.
Melamine cyanurate is commonly sold as a flame-retardant compound. Although effective as a flame-retardant, high levels of this material oftentimes severely reduce the flowability of molding compounds. As a result, it has generally been considered impractical to incorporate melamine cyanurate into molding compounds at appropriate levels to achieve both adequate flame retardancy and flowability.
Unfortunately, reducing the amount of the flame-retardant to address the flowability issues compromises flame retardance, with the resulting molding compounds failing to meet the flame-retardance standard, UL94 V—O rating.
It would be desirable to provide new flame-retardant molding compositions that overcome these, while providing commercially acceptable physical properties. Moreover, it would be desirable to provide molding compositions with excellent stress characteristics such as low warpage and shrinkage with improved flowability upon curing.